Familial aggregation of human susceptibility to co- and multiple helminth infections in a population from the Poyang Lake region, China.

Abstract

Human helminthiases are common in China, especially in rural areas where sanitation conditions are poor. Co- and multiple infections with helminths are strikingly frequent. A cross-sectional parasitological and questionnaire survey was carried out in a population of 3205 individuals belonging to 498 families from five villages in the Poyang Lake region, Jiangxi Province, China, to assess their helminth infection status and to collect information on risk factors for infection. The prevalences for Ascaris lumbricoides, Schistosoma japonicum and Trichuris trichiura were 30.9%, 15.7% and 47%, respectively. Hookworm infection prevalence was low (0.7%). A significant association was observed between A. lumbricoides and T. trichiura infection, and also between S. japonicum and T. trichiura infection. Variance components analysis was undertaken to investigate the aggregation of S. japonicum and the soil-transmitted helminths, A. lumbricoides and T. trichiura. While A. lumbricoides was found to aggregate only at a household level, T. trichiura was shown to cluster predominantly in families. Both genetic and household effects were found to be important in determining the risk of infection with S. japonicum. Variance components analysis for A. lumbricoides/T. trichiura co-infections indicated a significant domestic environmental effect, attributable for 32.7% of the co-infection risk. Aggregation of S. japonicum/T. trichiura co-infection was also observed at a household level. The risk of infection with multiple helminth species, although mainly environmentally influenced, was also shown to have significant involvement of genetic and household components. The results of this study indicate that a shared household is a major contributing risk factor for helminth co-infections and emphasises the need for increased standards of sanitation and hygiene to prevent parasite transmission. Further, the results suggest that susceptibility to one helminth infection is not completely independent of another, and that there exist common genetic factors underlying infection with multiple helminth species.

Full text

Page 1

Familial aggregation of human susceptibility to co- and multiple
helminth infections in a population from the Poyang Lake region,
China
Magda K. Ellisa,⁎, Giovanna Rasoa, Yue-Sheng Lia,b, Zhu Rongc, Hong-Gen Chend, and
Donald P. McManusa
aMolecular Parasitology Laboratory, Australian Centre for International and Tropical Health and
Nutrition, The Queensland Institute of Medical Research and The University of Queensland, 300
Herston Road, Herston, Brisbane, Qld 4029, Australia.
bHunan Institute of Parasitic Diseases, WHO Collaborating Centre for Research and Control on
Schistosomiasis in Lake Region, Yueyang, Hunan Province, PR China.
cNational Institute of Parasitic Diseases, Chinese Centre for Disease Control and Prevention, 207
Rui-Jin Road, Shanghai 200025, PR China.
dJiangxi Provincial Institute of Parasitic Diseases, 330046 Nanchang, PR China.
Abstract
Human helminthiases are common in China, especially in rural areas where sanitation conditions are
poor. Co- and multiple infections with helminths are strikingly frequent. A cross-sectional
parasitological and questionnaire survey was carried out in a population of 3205 individuals
belonging to 498 families from five villages in the Poyang Lake region, Jiangxi Province, China, to
assess their helminth infection status and to collect information on risk factors for infection. The
prevalences for Ascaris lumbricoides, Schistosoma japonicum and Trichuris trichiura were 30.9%,
15.7% and 47%, respectively. Hookworm infection prevalence was low (0.7%). A significant
association was observed between A. lumbricoides and T. trichiura infection, and also between S.
japonicum and T. trichiura infection. Variance components analysis was undertaken to investigate
the aggregation of S. japonicum and the soil-transmitted helminths, A. lumbricoides and T.
trichiura. While A. lumbricoides was found to aggregate only at a household level, T. trichiura was
shown to cluster predominantly in families. Both genetic and household effects were found to be
important in determining the risk of infection with S. japonicum. Variance components analysis for
A. lumbricoides/T. trichiura co-infections indicated a significant domestic environmental effect,
attributable for 32.7% of the co-infection risk. Aggregation of S. japonicum/T. trichiura co-infection
was also observed at a household level. The risk of infection with multiple helminth species, although
mainly environmentally influenced, was also shown to have significant involvement of genetic and
household components. The results of this study indicate that a shared household is a major
contributing risk factor for helminth co-infections and emphasises the need for increased standards
of sanitation and hygiene to prevent parasite transmission. Further, the results suggest that
© 2007 Elsevier Ltd
This document may be redistributed and reused, subject to certain conditions.
⁎Corresponding author. Tel.: +61 7 33620405; fax: +61 7 33620104. Magda.Ellis@qimr.edu.au.
This document was posted here by permission of the publisher. At the time of deposit, it included all changes made during peer review,
copyediting, and publishing. The U.S. National Library of Medicine is responsible for all links within the document and for incorporating
any publisher-supplied amendments or retractions issued subsequently. The published journal article, guaranteed to be such by Elsevier,
is available for free, on ScienceDirect.
Sponsored document from
International Journal for
Parasitology
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 2

susceptibility to one helminth infection is not completely independent of another, and that there exist
common genetic factors underlying infection with multiple helminth species.
Keywords
Variance components analysis; Schistosoma japonicum; Ascaris lumbricoides; Trichuris trichiura;
Poly-helminth infections
1  Introduction
Schistosomiasis and soil transmitted helminths (STHs) are estimated to account for more than
40% of the total tropical parasite burden, excluding malaria (WHO, 1999). Indeed, STHs
(Ascaris lumbricoides, hookworm (Ancylostoma, Necator) and Trichuris trichiura) are among
the most common cosmopolitan geo-helminths, and are widely distributed worldwide,
including China (Hotez et al., 1997). Although there is no systematic control of STHs currently
operating in China, a recent national survey of the important human parasitic diseases
(Anonymous, 2005) indicated a reduction in prevalence of STHs of 61%, 71% and 74% for
A. lumbricoides, hookworm and T. trichiura, respectively, since a previous national survey in
1990. Schistosoma japonicum is highly endemic to southern China. Due to the significant level
of morbidity observed in chronically infected individuals, schistosomiasis control has been a
major public health focus of Chinese authorities for more than five decades. Despite initial
successes, which saw numbers of infected individuals decrease by over 90%, there are still an
estimated 880,000 people afflicted by schistosomiasis in endemic foci (Li and Cai, 2004).
Given the high prevalence of STHs and S. japonicum, it is inevitable that concurrent infections
occur. Indeed, several studies across Southeast Asia, South America and Sub-Saharan Africa,
have described populations having significant helminth infection prevalence, with individuals
harbouring multiple helminth infections being the norm (Booth et al., 1996; Guignard et al.,
2000; Lili et al., 2000; Keiser et al., 2002; Waikagul et al., 2002; Tchuem Tchuenté et al., 2003;
Raso et al., 2004a,b; Zhou et al., 2005).
A characteristic feature of helminth infections is an over-dispersed distribution of observed
faecal egg counts whereby a minority of individuals account for the majority of the parasite
burden (Bradley and May, 1978; Anderson and May, 1985). Further, individuals who
experience high intensity of infection have been shown to aggregate in families. Significant
clustering at both a household and familial level was demonstrated in a population endemic
for A. lumbricoides in Nepal (Williams-Blangero et al., 1999); the heritable component in this
study accounted for ∼40% of the variation observed in faecal egg counts. Predisposition to T.
trichiura infection has also been shown to aggregate in families (Chan et al., 1994) and
quantitative methods estimated a strong heritable component (25.4%) underlying susceptibility
to T. trichiura infection intensity (Williams-Blangero et al., 2002a). Genetic susceptibility to
infection and infection intensity has been observed in populations living in areas endemic for
Schistosoma mansoni (Bethony et al., 2001, 2002a) and S. japonicum (Ellis et al., 2006).
Although these genetic studies investigated the clustering of single helminth infections at a
familial level, nothing is known of the genetics underlying multiple helminth infections. Here
we describe features of a Chinese population residing in an area co-endemic for STHs and S.
japonicum previously studied by Ellis et al. (2006). We provide insight into the factors
contributing to the susceptibility to single helminth infections in this population, using a
variance components approach. In addition, we believe we provide the first evidence of
possible genetic effects underlying the susceptibility to co- and multiple helminth infections
in humans.
Ellis et al. Page 2
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 3

2 
2.1 
 Materials and methods
 Study population
The study population comprised five administrative villages from the Poyang Lake region in
Jiangxi Province, China (Ellis et al., 2006). The five villages are located at different positions
on Poyang Lake; geo-coordinates of each village are as follows: Dingshan (E115.97 N29.12),
Xindong (E116.69 N28.85), Fuqian (E116.42 N28.89), Aiguo (E116.37 N28.73) and Yu Feng
(E116.13 N28.51). The total population consisted of 3682 individuals. Each administrative
village was comprised of four to eight smaller ‘natural villages’. Each individual was allocated
a personal identification code (PID) comprised of their administrative village code, natural
village code, household and house member code. Children under the age of 5 years were not
included.
2.2  Questionnaires for the collection of demographic, treatment history and pedigree data
All individuals in the study were interviewed using an existing, validated questionnaire (Ross
et al., 1997). Demographic information for age, sex, education and occupation was collected
as well as history of schistosomiasis, including past infection and treatment data, and water
contact. Water contact was assessed by mode and frequency of contact for spring, summer and
autumn (Ross et al., 1998b).
A further questionnaire was developed and used to collect pedigree data. All individuals in the
study were interviewed to identify all immediate biological relatives, i.e. parents, siblings and
children. Family member PIDs were recorded as well as relationship, name, sex and date of
birth of the relative to ensure correct identification. Extended families were identified using
the questionnaires and family trees were created for each. Five hundred and thirty-one
individuals were absent from the study (deceased or non-permanent residents) but were still
included as dummy relatives in order to link families further. In-laws, who were not
biologically related to anyone else in a family (i.e. had no children), were not included, and
thus were not used in the household analysis. When inconsistencies were identified between
the questionnaires of relatives (i.e. a wrong PID code of a relative), the information was
checked, where possible, or omitted from the final data analysis. The final data comprised a
total of 3205 individuals, belonging to a total of 498 families which ranged in size from three
to 188 and spanned as many as four generations.
2.3  Parasitological examination
All individuals from the study population were asked to provide two stool samples for
parasitological examination using the Kato–Katz thick smear technique (Katz et al., 1972).
The second stool was collected at least 72 h after the first. In order to maximise sensitivity of
the technique (Ross et al., 1998a; Berhe et al., 2004), three slides were prepared from each
stool sample (Ross et al., 1998a; Li et al., 1997, 2003). Each slide was read blind by experienced
microscopists for the presence of S. japonicum and STHs (A. lumbricoides, hookworm and T.
trichiura) eggs and 10% of slides were re-read for quality control by a senior microscopist
(Utzinger et al., 2000; Raso et al., 2004a,b). Positive infection was defined for each parasite
species if there was an egg count of one or more on any slide. This study was primarily
undertaken to investigate the genetics underlying susceptibility to S. japonicum infection in
this population (Ellis et al., 2006); as a result, intensity of infection (eggs per gram of faeces;
EPG) was recorded only for S. japonicum and not for the STH infections. Infection intensity
was calculated for S. japonicum as EPG. Individuals with S. japonicum infection were divided
into five categories depending on their infection intensity: low infection (1–4 EPG); minor
infection (5–12 EPG); moderate infection (13–60 EPG); high infection (61–132 EPG) and
highest infection (133+ EPG).
Ellis et al. Page 3
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 4

2.4  Data management and statistical analysis
All collected parasitological and questionnaire data were double-entered into an Access
database and cross-checked. Investigation of prevalence estimates, parasite distributions and
associations was carried out in SPSS 13.0. SOLAR, a Unix-based software program, which
was also used to carry out the variance components analysis (Duggirala et al., 1997; Almasy
and Blangero, 1998), the models for which were described previously (Ellis et al., 2006).
Family data was coded and indexed according to the file format requirements of the SOLAR
software package. Both a natural log transformation (ln(EPG + 1)) and a blom transformation
were applied to normalise the distribution of EPG. However, skewness and kurtosis remained
significantly different from zero (P < 0.001). Variance components analysis is based on a
regression model whereby the phenotype is assumed to be normally distributed. If this
assumption is violated, the standard errors of the parameters are underestimated and can result
in type I errors, thus leading to incorrect conclusions on the model of best fit. As a result, only
the binary phenotype (infected versus uninfected) and not EPG was used for S. japonicum.
Aggregation of multiple helminth infections was investigated using the number of infections
with any parasite as the phenotype. For this purpose a variable was generated, with values
ranging from 0 (having no infection) to 3 (harbouring three parasite species), and was treated
as a continuous variable with a normal distribution (skewness = 0.477; kurtosis = −0.8). If the
phenotype under investigation is dichotomous, and therefore cannot be normally distributed,
a multifactorial threshold model is used that assumes: (i) that there are several factors that are
involved in the disorder in question; (ii) that the effect of each individual factor is small but
that the effects of each factor are additive; and (iii) once the additive effect of these factors
pass a critical threshold, an individual becomes affected. This implies that the underlying
liability of affection is normally distributed but once the threshold is reached, one becomes
affected and the phenotype is dichotomous.
Briefly, in a variance components analysis, the total residual variance can be expressed as the
sum of the additive genetic , common environment
environment variances.
and individual-specific
Environmental, polygenic (additive genetic effects) and common environment (in this case
household) effects can thus be estimated according to the parameter constraints specified in
the model. The general household/polygenic model estimates all parameters, whereas the
household and the polygenic models are specified by fixing
Heritability can be estimated as simply the standardised value of the additive genetic effect
. An advantage of using SOLAR is its ability to incorporate covariate effects into the models
to account for potential confounding factors and a Kullback–Leibler R-squared value is
estimated to indicate the effect size of the covariates in the model.
and at zero, respectively.
Here, all significant covariates (P < 0.05) identified in the bivariate logistic regression were
included in the models as well as administration village (village effects) to account for any
potential geographical differences between the study sites. The software implements a
maximum likelihood method which provides a log-likelihood estimate for each model. As all
models are nested on the household/polygenic model, they can then be compared with one
another using a likelihood ratio test (LRT) which calculates a chi square value to be twice the
difference between the likelihoods with one degree of freedom. The significance of the
household and genetic effects can thus be tested individually (versus the sporadic model) and
in the presence of each other (versus household/polygenic model). The model of best fit was
selected as the model with most significant effects (<0.05).
Ellis et al.Page 4
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 5

2.5  Ethical considerations
Ethical approval for the study was granted by the ethics committees of Jiangxi Provincial
Institute of Parasitic Diseases, Chinese Center for Disease Control and Prevention; National
Institute of Parasitic Diseases, Chinese Center for Disease Control and Prevention, Shanghai;
Hunan Institute of Parasitic Diseases, Chinese Center for Disease Control and Prevention; and
the Queensland Institute of Medical Research, prior to commencement. Oral informed consent
was obtained from all adults and from parents or guardians of minors (over the age of 4 years)
who were involved in the project. Study participants identified as stool egg-positive for
schistosomiasis were treated with 40 mg/kg of praziquantel, the current dosage recommended
by the WHO. Subjects infected with STHs were advised to seek treatment from a local doctor
or hospital.
3 
3.1 
 Results
 Parasite infections
Among the five administrative villages, the observed prevalences for A. lumbricoides and T.
trichiura ranged from 0% to 38.8% (mean 30.9%), and from 2.4% to 58.1% (mean 47%),
respectively, whereas the S. japonicum prevalence ranged from 12.2% to 20.9% (mean 15.7%).
Hookworm infection prevalence was low (0.7%), and was not considered in further analysis.
The difference in helminth prevalence between the administrative villages was highly
significant (χ2; P < 0.001). Of the total population, 61.7% harboured at least one helminth
species and 27.8% were identified as harbouring at least two parasite species. Again,
differences between the mean number of helminth species harboured by individuals among
villages were highly significant (Kruskal–Wallis test; P < 0.001) although the overall
distribution of the number of parasite species among individuals was normal.
Fig. 1 illustrates the mean number of helminth parasite species in individuals infected with S.
japonicum. Individuals with low S. japonicum infection intensity were shown to have a higher
mean number of parasite species. When a higher intensity of infection was experienced, the
number of other helminth species decreased. The mean number of parasite species increased
again in those with highest egg counts. These differences between the egg count groups were
not significant (Kruskal–Wallis test; P = 0.25).
3.2  Parasite–parasite associations and covariates associated with infection status
There were no associations observed between S. japonicum and A. lumbricoides infections.
However, there was a significant (P = 0.03) increase in risk of T. trichiura infection in an
individual infected with S. japonicum (Odds Ratio = 1.28; 95% Confidence Interval
(CI) = 1.03–1.60). Further, there was a strong association between A. lumbricoides infection
and T. trichiura infection (P < 0.001; Odds Ratio = 4.43; 95% CI = 3.67–5.34).
Table 1 shows associations and risk factors for single, co- and multiple helminth infections.
The youngest age group (5–14 years old) was at a significantly lower risk of infection with S.
japonicum compared with older age groups. Gender, water contact and the number of past
treatments were highly significant risk factors for S. japonicum infection. There was no age or
gender association with A. lumbricoides infection; however, there was a significant association
with water contact. Age and water contact were significant risk factors for T. trichiura infection.
The highest T. trichiura prevalence (63.4%) was observed in the 5–14 years age group.
3.3  Variance components analysis of individual helminth infection
Four models were tested for each individual helminth infection: environmental, household,
polygenic and general (household and polygenic). All models were compared with each other
using the LRT. The favoured models for each infection, with corresponding parameter
Ellis et al. Page 5
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 6

estimates, are provided in Table 2. A household model was favoured for A. lumbricoides
indicating that 31.7% of risk of infection was attributable to domestic environment. No
clustering of infection was observed at a familial level. T. trichiura infection was best described
by the polygenic model; heritability was strong (29.9%) and significant (P < 0.05). Significant
heritability was also estimated for S. japonicum infection (23.7%) as well as a strong household
component (24.7%). The Kullback–Leibler R2 value estimated for all models was low,
indicating that the covariates did not account for much of the variation in the models.
3.4  Variance components analysis of co- and multiple helminth infections
Given the strong association observed between A. lumbricoides and T. trichiura infection, a
variance components analysis was undertaken to determine the factors underlying risk of
having both infections concurrently (Table 3). Household effects were strongly significant
(P < 0.05) and accounted for 32.7% of the risk of being infected with both parasites.
The association between S. japonicum and T. trichiura infection was also investigated. Again
a household model was favoured with a strong effect size of 43.16% (P < 0.05; Table 3).
The variance components analysis estimated a heritability of 16.32% and a household effect
of 8.8% for multiple helminthic infections. When both the household model and the polygenic
model were compared to the general model (Table 3), both genetic and shared household effects
remained significant in the presence of each other (P < 0.05).
4  Discussion
Helminth infections were common in this study population from the Poyang Lake region,
China, and one third of the participants were found to harbour at least two parasite species
concurrently. Hookworm prevalence was low and was not considered further in the analysis.
As frequently observed with S. japonicum infections, males were more likely to be infected
than females (Huang and Manderson, 1992, 2005; He et al., 1996). Water contact was also
shown to be a major contributing factor for S. japonicum infection, reflecting the transmission
characteristics of the parasite (Ross et al., 1998b; Li et al., 2000; Chen and Lin, 2004).
Interestingly, water contact was a significant risk factor for individual infection, co-infection
and multiple infections which would suggest water contamination as a common feature of the
transmission of S. japonicum, T. trichiura and A. lumbricoides. A significant association was
observed between S. japonicum and T. trichiura infection and between A. lumbricoides and
T. trichiura infection. The latter association could be largely due to the similarity in
transmission of these two nematode species which is closely related to poor hygiene behaviour
and the lack of adequate sanitation (Booth et al., 1996; Tchuem Tchuenté et al., 2003; Raso et
al., 2004a,b). The number of past treatments was also a significant indicator of current infection,
suggesting those who had a high number of previous treatments were more prone to re-
infection.
It is important to note that the results presented in Fig. 1 depict a trend as differences between
the egg count categories were not statistically significant. However, a recent study in Brazil in
individuals with concurrent S. mansoni and A. lumbricoides infection also indicated infection
intensity for each parasite was lower than in those experiencing a single infection; a hypothesis
was proposed suggesting that the similarity in the human immune response to both species
may account for the decrease in parasite burden (Fleming et al., 2006). In contrast, however,
our data (Fig. 1) suggest that those who appeared to be susceptible to extremely high levels of
S. japonicum intensities also appeared to be highly susceptible to co- or multiple parasite
infections, with many harbouring at least one other parasite. The association between very high
infection intensity and polyparasitism has been described previously (Tchuem Tchuenté et al.,
2003) and is suggestive that immunosuppression may be involved at this level of infection and
Ellis et al. Page 6
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 7

this is an area that deserves further exploration. Further work to collect egg count data for STHs
would also be of great interest to examine the correlations between different helminth infection
intensities and to investigate the effect of poly-helminth infections on the associated clinical
morbidity of individual infection.
Given the significant differences observed between helminth prevalences in the different
villages, a village effect was incorporated into all the variance components analyses. The
variance accounted for by other covariates in the model (Kullback–Leibler R2) was not shown
to be high in any of the models and thus had little impact on the final parameter estimates. The
variance components analysis for A. lumbricoides infection indicated household as a major
contributing risk factor. In contrast to a previous study by Williams-Blangero et al. (1999),
there did not appear to be any additive genetic effects involved. This is likely due to the binary
phenotype used in this analysis, which decreased the power of the study to detect any
aggregation at a familial level. Despite the loss of power due to the use of a binary phenotype
(infection versus non-infection), significant heritability (29.9%) was still observed for T.
trichiura infection although there was no aggregation at the household level. These findings
are similar to those of Williams-Blangero et al. (2002a). Common domestic environment did
appear to play a role in the risk of S. japonicum infection as well as a genetic involvement.
Heritability remained high at 23.7% and was a significant risk factor; household was found to
account for 24.7% of the risk of infection. These results are comparable with those of studies
on S. mansoni (Bethony et al., 2001, 2002a).
The significant association identified between A. lumbricoides and T. trichiura infection was
investigated by a variance components analysis in order to identify factors contributing to the
risk of having a co-infection. The household model was found to be the most parsimonious
and the household effect was surprisingly high (33.6%). This again may be explained, in part,
by the commonality in the mode of transmission but, more notably, it could be a reflection of
the socio-economic status of a particular household and the sanitation and hygiene conditions
prevailing. Although data on household-specific risk factors were not collected (and merit
further investigation) the results suggest that while mass chemotherapy is fundamental in the
control of geo-helminths, an emphasis on the development of sanitation and clean water
facilities, improved health and hygiene awareness, and more targeted treatment of those living
in poor conditions is crucial to prevent transmission. Studies where sanitation and health
awareness in a population have been improved and followed over time have shown marked
decreases in prevalence that had been previously unattainable through chemotherapy alone
(Esrey et al., 1991; Narain et al., 2000; Asaolu and Ofoezie, 2003), thus emphasising the value
of using this combination method for control of STHs.
Variance components analysis of combined S. japonicum/T. trichiura infection also favoured
a household model with an effect size of 43.2%. Despite the difference in transmission of these
two parasites, water contact was a significant risk factor for both infections, again probably
reflecting the lifestyle of the study communities. Although faecal matter is no longer used
routinely as fertiliser in China, indiscriminate defecation is not uncommon (Bethony et al.,
2002b; Davis et al., 2002). This suggests that control measures implemented for STHs in these
villages would also reduce S. japonicum transmission. Further, a recent study of S. mansoni
endemic communities in Brazil, has shown that adults in shared households of school children
who were infected were also likely to infected (Massara et al., 2006). Targeting ‘infected
households’ is thus an effective, rapid and practical method of identifying and treating
helminth-infected adults in the community.
This investigation has shown that although the risk of infection with multiple helminth parasites
was largely environmentally influenced, the genetic component was significant, accounting
for 16.3% of the risk, and suggesting poly-helminth infections tends to aggregate in families.
Ellis et al. Page 7
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 8

This would indicate that there may be common genetic components involved in susceptibility
to multiple helminthic infections and that susceptibility to one parasite is not necessarily
independent of susceptibility to another.
Overall, relatively few genetic studies have been conducted for susceptibility to helminth
infections in human populations. Segregation and linkage studies have identified regions of
interest for both S. mansoni (Marquet et al., 1996, 1999; Muller-Myhsok et al., 1997) and A.
lumbricoides infections (Williams-Blangero et al., 2002b), although, to date, no such studies
have been carried out on T. trichiura or S. japonicum.
We believe this study represents the first insight into the aggregation of multiple helminth
infections in humans, suggesting there are common genetic factors involved in susceptibility
to different parasitic helminth species, and it provides a foundation for future investigations of
the genetics underlying this phenomenon. This will enable studies to be undertaken to identify
genes or gene loci associated with susceptibility to helminth infection(s) and to identify
possible gene–gene and gene–environment interactions involved in single and multiple
infection events. As such, this genetic analysis may result in the possible identification of new
drug targets and more targeted treatment of individuals susceptible to infection with parasitic
helminths.
Acknowledgments
This study was supported by the National Institute of Allergy and Infectious Diseases (NIAID) (Tropical Medicine
Research Center grant 1 P 50AI-39461) and a National Health and Medical Research Council of Australia and
Wellcome Trust (UK) International Collaborative Research Grants Scheme Award.
References
Almasy L. Blangero J. Multipoint quantitative trait linkage analysis in general pedigrees. Am. J. Hum.
Gen. 1998;62:1112–1198.
Anderson R.M. May R.M. Helminth infections of humans: mathematical models, population dynamics,
and control. Adv. Parasitol. 1985;24:1–101. [PubMed: 3904343]
Anonymous, 2005. Coordinating office of the national survey on the important human parasitic diseases.
A national survey on current status of the important parasitic diseases in human population. Zhongguo
Ji Sheng Chong Xue Yu Ji Sheng Chong Bing Za Zhi. 23, 332–340.
Asaolu S.O. Ofoezie I.E. The role of health education and sanitation in the control of Helminth infections.
Acta Trop. 2003;86:283–294. [PubMed: 12745145]
Berhe N. Medhin G. Erko B. Smith T. Gedamu S. Bereded D. Moore R. Habte E. Redda A. Gebre-Michael
T. Gundersen S.G. Variations in helminth faecal egg counts in Kato–Katz thick smears and their
implications in assessing infection status with Schistosoma mansoni. Acta Trop. 2004;92:205–212.
[PubMed: 15533288]
Bethony J. Gazzinelli A. Lopes A. Pereira W. Alves-Oliveira L. Willams-Blangero S. Blangero J. Loverde
P. Correa-Oliveira R. Genetic epidemiology of fecal egg excretion during Schistosoma mansoni
infection in an endemic area in Minas Gerais, Brazil. Mem. Inst. Oswaldo Cruz. 2001;96(Suppl.):49–
55. [PubMed: 11586426]
Bethony J. Williams J.T. Blangero J. Kloos H. Gazzinelli A. Soares-Filho B. Coelho L. Alves-Fraga L.
Williams-Blangero S. Loverde P.T. Correa-Oliveira R. Additive host genetic factors influence fecal
egg excretion rates during Schistosoma mansoni infection in a rural area in Brazil. Am. J. Trop. Med.
Hyg. 2002;67:336–343. [PubMed: 12452486]
Bethony J. Chen J. Lin S. Xiao S. Zhan B. Li S. Xue H. Xing F. Humphries D. Yan W. Chen G. Foster
V. Hawdon J.M. Hotez P.J. Emerging patterns of hookworm infection: influence of aging on the
intensity of Necator infection in Hainan Province, People’s Republic of China. Clin. Infect. Dis.
2002;35:1336–1344. [PubMed: 12439796]
Ellis et al. Page 8
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 9

Booth M. Guyatt H.L. Li Y. Tanner M. The morbidity attributable to Schistosoma japonicum infection
in 3 villages in Dongting Lake region, Hunan province, PR China. Trop. Med. Int. Health. 1996;1:646–
654. [PubMed: 8911449]
Bradley D.J. May R.M. Consequences of helminth aggregation for the dynamics of schistosomiasis.
Trans. R. Soc. Trop. Med. Hyg. 1978;72:262–273. [PubMed: 675786]
Chan L. Bundy D.A. Kan S.P. Aggregation and predisposition to Ascaris lumbricoides and Trichuris
trichiura at the familial level. Trans. R. Soc. Trop. Med. Hyg. 1994:8846–8848.
Chen H. Lin D. The prevalence and control of schistosomiasis in Poyang Lake region, China. Parasitol.
Int. 2004;53:115–125. [PubMed: 15081943]
Davis G.M. Wu W.P. Chen H.G. Liu H.Y. Guo J.G. Lin D.D. Lu S.B. Williams G. Sleigh A. Feng Z.
McManus D.P. A baseline study of importance of bovines for human Schistosoma japonicum
infections around Poyang Lake, China: villages studied and snail sampling strategy. Am. J. Trop.
Med. Hyg. 2002;66:359–371. [PubMed: 12164289]
Duggirala R. Williams J.T. Williams-Blangero S. Blangero J. A variance component approach to
dichotomous trait linkage analysis using a threshold model. Genet. Epidemiol. 1997;14:987–992.
[PubMed: 9433612]
Ellis M.K. Li Y. Rong Z. Chen H. McManus D.P. Familial aggregation of human infection with
Schistosoma japonicum in the Poyang Lake region, China. Int. J. Parasitol. 2006;36(1):71–77.
[PubMed: 16321389]
Esrey S.A. Potash J.B. Roberts L. Shiff C. Effects of improved water supply and sanitation on ascariasis,
diarrhoea, dracunculiasis, hookworm infection, schistosomiasis, and trachoma. Bull. World Health
Organ. 1991;69:609–621. [PubMed: 1835675]
Fleming F.M. Brooker S. Geiger S.M. Caldas I.R. Correa-Oliveira R. Hotez P.J. Bethony J.M. Synergistic
associations between hookworm and other helminth species in a rural community in Brazil. Trop.
Med. Int. Health. 2006;11:56–64. [PubMed: 16398756]
Guignard S. Arienti H. Freyre L. Lujan H. Rubinstein H. Prevalence of enteroparasites in a residence for
children in the Cordoba Province, Argentina. Eur. J. Epidemiol. 2000;16:287–293. [PubMed:
10870946]
He N. Yuan H.C. Zhang S.J. Jiang Q.W. A quantitative study on human water contact in a hilly rural
endemic area of Schistosoma japonicum. Chin. J. Schisto. Contr. 1996;8:1–5.
Hotez P.J. Zheng F. Long-qi X. Ming-gang C. Shu-hua X. Shu-xian L. Blair D. McManus D.P. Davis
G.M. Emerging and reemerging helminthiases and the public health of China. Emerg. Infect. Dis.
1997;3:303–310. [PubMed: 9284374]
Huang Y. Manderson L. Schistosomiasis and the social patterning of infection. Acta Trop. 1992;51:175–
194. [PubMed: 1359746]
Huang Y.X. Manderson L. The social and economic context and determinants of schistosomiasis
japonica. Acta Trop. 2005;96:223–231. [PubMed: 16202596]
Katz N. Chaves A. Pellegrino J. A simple device for quantitative stool thick-smear technique in
Schistosomiasis mansoni. Rev. Inst. Med. Trop. Sao Paulo. 1972;14:397–400. [PubMed: 4675644]
Keiser J. N’Goran E.K. Traore M. Lohourignon K.L. Singer B.H. Lengeler C. Tanner M. Utzinger J.
Polyparasitism with Schistosoma mansoni, geohelminths, and intestinal protozoa in rural Cote
d’Ivoire. J. Parasitol. 2002;88:461–466. [PubMed: 12099412]
Li Y.S. Ross A.G. Yu D.B. Li Y. Williams G.M. McManus D.P. An evaluation of Schistosoma japonicum
infections in three villages in the Dongting lake region of China. I. Prevalence, intensity and morbidity
before the implementation of adequate control strategies. Acta Trop. 1997;68:77–91. [PubMed:
9352004]
Li Y.S. Sleigh A.C. Williams G.M. Ross A.G. Forsyth S.J. Tanner M. McManus D.P. Measuring exposure
to Schistosoma japonicum in China. III. Activity diaries, snail and human infection, transmission
ecology and options for control. Acta Trop. 2000;75:279–289. [PubMed: 10838211]
Li Y.S. He Y.K. Zeng Q.R. McManus D.P. Epidemiological and morbidity assessment of Schistosoma
japonicum infection in a migrant fisherman community, the Dongting Lake region, China. Trans. R.
Soc. Trop. Med. Hyg. 2003;97:177–181. [PubMed: 14584373]
Li Y.S. Cai K.P. The epidemic trend and challenges for schistosomiasis in China. Chinese J. Epidemiol.
2004;25:553–554.
Ellis et al. Page 9
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 10

Lili Z. Bingxiang Z. Hong T. Shuhua X. Hotez P. Bing Z. Yangzhong L. Ying L. Haichou X. Hawdon
J. Hui Y. Huizhen W. Zhen F. Epidemiology of human geohelminth infections (ascariasis, trichuriasis
and necatoriasis) in Lushui and Puer Counties, Yunnan Province, China. Southeast Asian J. Trop.
Med. Public Health. 2000;31:448–453. [PubMed: 11288999]
Marquet S. Abel L. Hillaire D. Dessein H. Kalil J. Feingold J. Weissenbach J. Dessein A.J. Genetic
localization of a locus controlling the intensity of infection by Schistosoma mansoni on chromosome
5q31-q33. Nat. Genet. 1996;14:181–184. [PubMed: 8841190]
Marquet S. Abel L. Hillaire D. Dessein A. Full results of the genome-wide scan which localises a locus
controlling the intensity of infection by Schistosoma mansoni on chromosome 5q31-q33. Eur. J.
Hum. Genet. 1999;7:88–97. [PubMed: 10094195]
Massara C.L. Peixoto S.V. Enk M.J. da Silva B.H. dos Santos C.O. Sakurai E. Schall V. Evaluation of
an improved approach using residences of schistosomiasis-positive school children to identify
carriers in an area of low endemicity. Am. J. Trop. Med. Hyg. 2006;74:495–499. [PubMed:
16525112]
Muller-Myhsok B. Stelma F.F. Guisse-Sow F. Muntau B. Thye T. Burchard G.D. Gryseels B. Horstmann
R.D. Further evidence suggesting the presence of a locus, on human chromosome 5q31-q33,
influencing the intensity of infection with Schistosoma mansoni. Am. J. Hum. Genet. 1997;61:452–
454. [PubMed: 9311752]
Narain K. Rajguru S.K. Mahanta J. Prevalence of Trichuris trichiura in relation to socio-economic &
behavioural determinants of exposure to infection in rural Assam. Indian J. Med. Res. 2000;112:140–
146. [PubMed: 11200680]
Raso G. N’Goran E.K. Toty A. Luginbühl A. Adjoua C.A. Tian-Bi N.T. Bogoch I.I. Vounatsou P. Tanner
M. Utzinger J. Efficacy and side effects of praziquantel against Schistosoma mansoni in a community
of western Côte d’Ivoire. Trans. R. Soc. Trop. Med. Hyg. 2004;98:18–27. [PubMed: 14702835]
Raso G. Luginbühl A. Adjoua C.A. Tian-Bi N.T. Silué K.D. Matthys B. Vounatsou P. Wang Y. Dumas
M.E. Holmes E. Singer B.H. Tanner M. N’Goran E.K. Utzinger J. Multiple parasite infections and
their relationship to self-reported morbidity in a community of rural Côte d’Ivoire. Int. J. Epidemiol.
2004;33:1092–1102. [PubMed: 15256525]
Ross A.G. Yuesheng L. Sleigh A.S. Yi L. Williams G.M. Wu W.Z. Xinsong L. Yongkang H. McManus
D.P. Epidemiologic features of Schistosoma japonicum among fishermen and other occupational
groups in the Dongting Lake region (Hunan Province) of China. Am. J. Trop. Med. Hyg.
1997;57:302–308. [PubMed: 9311640]
Ross A.G. Li Y. Sleigh A.C. Williams G.M. McManus D.P. Faecal egg aggregation in humans infected
with Schistosoma japonicum in China. Acta Trop. 1998;70:205–210. [PubMed: 9698267]
Ross A.G. Li Y. Sleigh A.C. Williams G.M. Hartel G.F. Forsyth S.J. Yi L. McManus D.P. Measuring
exposure to S. japonicum in China. I. Activity diaries to assess water contact and comparison to other
measures. Acta Trop. 1998;71:213–228. [PubMed: 9879732]
Tchuem Tchuenté L.A. Behnke J.M. Gilbert F.S. Southgate V.R. Vercruysse J. Polyparasitism with
Schistosoma haematobium and soil-transmitted helminth infections among school children in Loum,
Cameroon. Trop. Med. Int. Health 2003;8:975–986. [PubMed: 14629763]
Utzinger J. N’Goran E.K. Tanner M. Lengeler C. Simple anamnestic questions and recalled water-contact
patterns for self-diagnosis of Schistosoma mansoni infection among schoolchildren in western Cote
d’Ivoire. Am. J. Trop. Med. Hyg. 2000;62:649–655. [PubMed: 11289679]
Waikagul J. Krudsood S. Radomyos P. Radomyos B. Chalemrut K. Jonsuksuntigul P. Kojima S.
Looareesuwan S. Thaineau W. A cross-sectional study of intestinal parasitic infections among
schoolchildren in Nan Province, Northern Thailand. Southeast Asian J. Trop. Med. Public Health.
2002;33:218–223. [PubMed: 12236415]
Williams-Blangero S. Subedi J. Upadhayay R.P. Manral D.B. Rai D.R. Jha B. Robinson E.S. Blangero
J. Genetic analysis of susceptibility to infection with Ascaris lumbricoides. Am. J. Trop. Med. Hyg.
1999;60:921–926. [PubMed: 10403321]
Williams-Blangero S. McGarvey S.T. Subedi J. Wiest P.M. Upadhayay R.P. Rai D.R. Jha B. Olds G.R.
Guanling W. Blangero J. Genetic component to susceptibility to Trichuris trichiura: evidence from
two Asian populations. Genet Epidemiol. 2002;22:254–264. [PubMed: 11921085]
Ellis et al. Page 10
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 11

Williams-Blangero S. VandeBerg J.L. Subedi J. Aivaliotis M.J. Rai D.R. Upadhayay R.P. Jha B. Blangero
J. Genes on chromosomes 1 and 13 have significant effects on Ascaris infection. Proc. Natl. Acad.
Sci. USA 2002;16(99):5533–5538. [PubMed: 11960011]
WHO 1999: Making a Difference. World Health Organisation, Geneva, Switzerland.
Zhou H. Ohtsuka R. He Y. Yuan L. Yamauchi T. Sleigh A.C. Impact of parasitic infections and dietary
intake on child growth in the schistosomiasis-endemic Dongting Lake Region, China. Am. J. Trop.
Med. Hyg. 2005;72:534–539. [PubMed: 15891126]
Ellis et al.Page 11
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 12

Fig. 1.
The number of helminth parasite species in individuals from the study population in the Poyang
Lake, China, infected with differing infection intensities of Schistosoma japonicum.
Ellis et al.Page 12
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.
Sponsored Document
Sponsored Document
Sponsored Document

Page 13

Sponsored Document
Sponsored Document
Sponsored Document
Ellis et al.Page 13
Table 1
Association of single, co- and multiple helminthic infections, in the Poyang Lake study population using bi-
variate logistic regression analysis
Parasite
N
Number
(%)
infected
individuals
Association Odds ratio95% confidence interval
P-valuea
Schistosoma japonicum
Age group
49746 (9.8) 5–1410.002
24645 (19.2) 15–242.201.41–3.43
369 60 (16.8) 25–341.87 1.24–2.82
53895 (18.4) 35–44 2.091.43–3.05
36561 (17.3) 45–541.93 1.28–2.91
43562 (14.8) 55+ 1.611.07–2.41
Sex
1032 123 (12.4) Females1 <0.001
1428 247 (18) Males 1.551.22–1.95
1670Water contact 1.491.24–1.79<0.001
2452Treatment number 1.031.01–1.05 <0.001
2460125 (5.1)
A. lumbricoides
1.170.93–1.480.19
2460193 (7.8)
Trichuris trichiura
1.281.03–1.600.03
Ascaris lumbricoides
Age group
497167 (33.6) 5–1410.08
24672 (29.3) 15–24 0.820.59–1.14
369 109 (29.5) 25–34 0.830.62–1.11
538 187 (34.8) 35–441.05 0.81–1.36
36598 (26.8) 45–540.730.54–0.98
435 125 (28.7) 55+0.800.60–1.05
Sex
1032 313 (30.3) Females10.57
1428 448 (31.4) Males 0.95 0.80–1.13
1670 Water contact1.421.22–1.65<0.001
2460 550 (22.3)
T. trichiura
4.433.67–5.34 <0.001
T. trichiura
Age group
497 315 (63.4) 5–141 <0.001
246 117 (47.6) 15–240.520.38–0.71
369 155 (42.0) 25–340.420.32–0.55
538249 (46.3) 35–440.500.40–0.64
365159 (43.6) 45–54 0.450.34–0.59
435182 (41.8) 55+0.420.32–0.54
Sex
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.

Page 14

Sponsored Document
Sponsored Document
Sponsored Document
Ellis et al. Page 14
Parasite
N
Number
(%)
infected
individuals
Association Odds ratio95% confidence interval
P-valuea
1032 511 (49.5) Females1 0.2
1428 669 (46.8) Males0.900.77–1.06
1670Water contact1.561.37–1.79 <0.001
A. lumbricoides/T. trichiura
Age group
497139 (28) 5–1410.07
24649 (19.9) 15–24 0.640.44–0.93
369 80 (21.7) 25–340.710.52–0.98
538129 (24) 35–440.810.62–1.07
36566 (18.1) 45–54 0.570.41–0.79
43586 (19.8) 55+0.64 0.47–0.86
Sex
1032 230 (22.3) Females1 0.93
1428 320 (58.2) Males1.010.83–1.22
1670Water contact 1.441.20–1.72 <0.001
S. japonicum/T. trichiura
Age group
49737 (7.8) 5–1410.38
24624 (10.1) 15–241.340.78–2.29
36927 (7.5) 25–340.970.58–1.62
53848 (9.2) 35–441.20 0.77–1.88
36531 (8.7) 45–54 1.130.69–1.86
435 25 (5.9) 55+ 0.74 0.44–1.25
Sex
103263 (6.3) Females1 <0.001
1428130 (9.4) Males1.531.12–2.10
1670Water contact2.201.61–3.02 <0.001
Multiple helminth species2450 Age group
0.001b
2460Sex
<0.001c
1670 Water contact
<0.001d
N = Total population with no missing data and infection phenotype.
aP-value based on likelihood ratio test.
bP-value based on Kruskal–Wallis test.
cP-value based on Student’s t-test.
dP-value based on linear regression.
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.

Page 15

Sponsored Document
Sponsored Document
Sponsored Document
Ellis et al. Page 15
Table 2
Summary of the favoured models of the variance components analysis for single, co- and multiple helminthic
infections adjusted for significant covariates
Infection variable Favoured modelStandardised parameter estimates
Kullback–Leibler R2⁎ ⁎
Likelihood
GeneticHousehold Environment
Ascaris lumbricoidesa
Household (0)0.3170.683 0.078
−1196.863
Trichuris trichiurab
Polygenic0.299 (0) 0.7010.054
−819.762
Schistosoma japonicumc
General 0.2370.2470.5160.023
−970.507
S. japonicum/T. trichiurad
Household (0)0.4320.5680.047
−381.677
A. lumbricoides/T. trichiurae
Household (0)0.3360.664 0.013
−988.420
Multiple helminth speciesf
Polygenic0.163 0.0880.749 0.003
−253.742
aCovariates included in the model: water contact, Trichuris and administration village.
bCovariates included in the model: age, water contact, A. lumbricoides, S. japonicum and administration village.
cCovariates included in the model: age, sex, water contact, treatment number, Trichuris and administration village.
dCovariates included in the model: sex, water contact and administration village.
eCovariates included in the model: water contact and administration village.
fCovariates included in the model: age, sex, water contact and administration village.
⁎The Kullback–Leibler R2 value denotes the proportion of variance due to the covariates in the models (excluding administration village).
Published as: Int J Parasitol. 2007 August ; 37(10-3): 1153–1161.